Electrode material, a battery electrode, method of producing them, nonaqueous electrolyte battery and battery pack

ABSTRACT

According to one embodiment, there is provided an electrode material. The electrode material includes an active material which includes a titanium oxide compound having a monoclinic titanium dioxide crystal structure. The electrode material further includes a compound which exists on the surface of the active material and has a trialkylsilyl group represented by the formula (I). 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2  and R 3 , which may be the same or different, respectively represent an alkyl group having 1 to 10 carbon atoms.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2010-212627, filed Sep. 22, 2010; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an electrode material,a battery electrode, a method of producing an electrode material, amethod of producing a battery electrode, a nonaqueous electrolytebattery and a battery pack.

BACKGROUND

A nonaqueous electrolyte battery using titanium oxide as the negativeelectrode has less possibility of the generation of lithium dendritethan a battery using a carbonaceous material because titanium oxide hasa higher Li-absorbing and releasing potential than that of thecarbonaceous material. Also, titanium oxide is resistant tothermorunaway because titanium oxide is ceramics. This is the reason whya nonaqueous electrolyte battery using titanium oxide as the negativeelectrode is highly safe. Especially, a spinel type lithium titanatehaving spinel structure is not varied in volume by a charge-dischargereaction and is therefore a promising material as a negative electrodeactive material having excellent cycle performance and high safety.However, a nonaqueous electrolyte battery using titanium oxide has theproblem that it has a low energy density. For example, the theoreticalcapacity of titanium dioxide having an anatase structure is about 160mAh/g and the theoretical capacity of lithium-titanium complex oxidehaving a spinel structure such as Li₄Ti₅O₁₂ is about 170 mAh/g.

In light of this situation, much attention is now focused on a titaniumoxide compound having a monoclinic titanium dioxide crystal structure.The reversible capacity of the titanium oxide compound having amonoclinic titanium dioxide crystal structure is about 240 mAh/g whichis a significantly higher than those of other titanium oxide compounds.

However, when the titanium oxide compound having a monoclinic titaniumdioxide crystal structure is used as the negative electrode, this posesa problem concerning a reduced cycle life.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a typical sectional view of a thin type nonaqueous electrolytebattery of an embodiment;

FIG. 2 is an enlarged sectional view of the part A in FIG. 1;

FIG. 3 is an exploded perspective view of a battery pack of anembodiment;

FIG. 4 is a block diagram showing an electric circuit of a battery packof FIG. 3; and

FIG. 5 is a graph showing the increase ratio in each resistance ofExamples 1 to 3 and Comparative Examples 1 to 4.

DETAILED DESCRIPTION

In general, according to one embodiment, there is provided an electrodematerial. The electrode material comprises an active material comprisinga titanium oxide compound having a monoclinic titanium dioxide crystalstructure. The electrode material further comprises a compound whichexists on the surface of the active material and has a trialkylsilylgroup represented by the formula (I).

wherein R¹, R² and R³, which may be the same or different, respectivelyrepresent an alkyl group having 1 to 10 carbon atoms.

According to another embodiment, there is provided a method of producingan electrode material, the method comprising dipping particle of atitanium oxide compound having a monoclinic titanium dioxide crystalstructure in a solution of a compound with a trialkylsilyl grouprepresented by the above formula (I) and separating the particle of thetitanium oxide compound to which the above compound with a trialkylsilylgroup is adhere from the solution.

According to another embodiment, there is provided a battery electrodecomprising an active material comprising a titanium oxide compoundhaving a monoclinic titanium dioxide crystal structure; and a compoundwith a trialkylsilyl group represented by the formula (I).

According to another embodiment, there is provided a method of producinga battery electrode, the method comprising preparing a slurry bydispersing an active material comprising a titanium oxide compoundhaving a monoclinic titanium dioxide crystal structure and a compoundwith a trialkylsilyl group represented by the formula (I) in a solvent.

According to a still another embodiment, there is provided a nonaqueouselectrolyte battery comprising a negative electrode, a positiveelectrode and a nonaqueous electrolyte. The negative electrode comprisesan active material comprising a titanium oxide compound having amonoclinic titanium dioxide crystal structure. The above nonaqueouselectrolyte comprises the compound having a trialkylsilyl grouprepresented by the above formula (I).

According to a still another embodiment, there is provided a batterypack comprising the above nonaqueous electrolyte battery.

The crystal structure of the monoclinic titanium dioxide belongsprimarily to the space group C2/m, showing a tunnel structure. Here, thecrystal structure of monoclinic titanium dioxide is referred to asTiO₂(B). Also, the titanium oxide compound having a crystal structure ofmonoclinic titanium dioxide is referred to as a titanium oxide compoundhaving a TiO₂(B) structure. Incidentally, the detailed crystal structureof TiO₂(B) is described in R. Marchand, L. Brohan, M. Tournoux, MaterialResearch Bulletin 15, 1129 (1980). The titanium oxide compound having aTiO₂(B) structure may be represented by the formula Li_(x)TiO₂ (0≦x≦1).In this case, x in the above formula is varied between 0 and 1 by acharge-discharge reaction.

It is considered that the theoretical capacity of a battery can beraised by using the titanium oxide compound having a TiO₂(B) structureas the active material because the titanium oxide compound has a hightheoretical capacity.

However, the titanium oxide compound having a TiO₂(B) structure is asolid acid and has a solid acid point and a hydroxyl group on itssurface and therefore has high reactivity with a nonaqueous electrolyte.For this, in a battery using the titanium oxide compound having aTiO₂(B) structure as the negative electrode active material, thenonaqueous electrolyte is decomposed along with a charge-dischargereaction, leading to the formation of excess inorganic or organiccoating film on the negative electrode. This results in the increasedresistance and reduced cycle life.

In a battery using a carbonaceous material or a spinel type lithiumtitanate as the negative electrode active material, a side-reactionbetween the negative electrode and the nonaqueous electrolyte can belimited by adding vinylene carbonate to the nonaqueous electrolyte. In abattery like this, this vinylene carbonate is reduction-decomposed onthe negative electrode to form a stable coating film on the negativeelectrode, whereby the excess formation of a coating film can belimited. However, in the battery using the titanium oxide compoundhaving a TiO₂(B) structure as the negative electrode active material,the side-reaction between the negative electrode and the nonaqueouselectrolyte cannot be limited even if vinylene carbonate is added, sothat the formation of a coating film cannot be limited. There aretherefore problems concerning increase in resistance and reduction incycle life.

The inventors have found that the increase in the resistance of abattery can be limited by adding a compound having a trialkylsilyl groupin any one of the active material, electrode and nonaqueous electrolyte.This reason is considered that excess formation of a coating film islimited since a stable coating film containing a trialkylsilyl group isproduced. Hereinafter, each embodiment will be explained in detail.

First Embodiment

According to this embodiment, there is provided an electrode materialcomprising an active material comprising a titanium oxide compoundhaving a TiO₂(B) structure and a compound which exists on the surface ofthe active material and has a trialkylsilyl group represented by theformula (I).

In the formula (I), R¹, R² and R³, which may be the same or different,respectively represent an alkyl group having 1 to 10 carbon atoms.

In such an electrode material, the compound having a trialkylsilyl groupis preferably existed in the surface of primary particle of the titaniumoxide compound. Here, the surface of the primary particle of thetitanium oxide compound includes the inside surface of pores existing onthe surface of the primary particle.

The electrode material according to this embodiment is comprised as theactive material in the electrode of a battery. In such a battery, thenonaqueous electrolyte is decomposed during an initial charge and thesubsequent charge-discharge, leading to the formation of a coating filmon the surface of the electrode material. However, since the compoundhaving a trialkylsilyl group exists in the electrode material, a coatingfilm containing a trialkylsilyl group is formed. This coating filmcontaining a trialkylsilyl group is stable and limits a side-reactionbetween the titanium oxide compound and the nonaqueous electrolyte,which makes it possible to limit the decomposition of the nonaqueouselectrolyte. Excess formation of a coating film is thereby limited, sothat an increase in the resistance of the battery can be limited, withthe result that the cycle life can be improved. An effect like this canbe obtained by using a compound having a trialkylsilyl group which hasan alkyl group having 10 or less carbon atoms. The alkyl group of thetrialkylsilyl group is preferably one having 1 to 3 carbon atoms.

The existence of the trialkylsilyl group on the surface of the electrodematerial can be detected by the time-of-flight type secondary ionmass-analysis method. Also, whether the trialkylsilyl group is containedor not in the coating film formed on the surface of the electrodematerial can be determined by analyzing the surface of the electrodematerial after charge-discharge according to the time-of-flight typesecondary ion mass-analysis method.

In the time-of-flight type secondary ion mass-analysis method, forexample, primary ions (for example, Bi₃ ⁺⁺) are applied to the surfaceof a sample under vacuum. Then, positively or negatively chargedsecondary ions are emitted from the surface of the sample. These emittedsecondary ions are detected by a detector positioned at a fixed distancefrom the sample. At this time, the time during which the secondary ionsaccelerated by the same energy reach the detector is a function of themass. For this, in the time-of-flight type secondary ion mass-analysismethod, the mass-distribution of secondary ions and organic materialsand inorganic materials existing on the surface of the sample can beidentified. Also, the amount of each material can be compared from itspeak intensity.

In the electrode material according to the embodiment, the peakintensity ratio (I₁/I₂) is preferably 1 or more when the surface of theelectrode material is measured by the time-of-flight type secondarymass-analysis method. Here, I₁ is the peak intensity of the peak derivedfrom Si(CH₃)₃ ⁺, and I₂ is the peak intensity of the peak derived fromTi⁺. The existence of the trialkylsilyl group in the electrode materialincreases the ratio (I₁/I₂) of peak intensities to 1 or more. Also, theexistence of the trialkylsilyl group in the coating film formed aftercharge-discharge also ensures a peak intensity ratio (I₁/I₂) of 1 ormore. Using the electrode material having a peak intensity ratio (I₁/I₂)of 1 or more, the increase of the resistance of the battery is morelimited and the cycle life is more improves. The peak intensity ratio(I₁/I₂) is preferably 10 or less because an excessive existence of thetrialkylsilyl group is a cause of increase in resistance.

The compound containing the trialkylsilyl group is, though not limitedto, preferably at least one selected from phosphoric acid compoundscontaining the trialkylsilyl group and boric acid compounds containingthe trialkylsilyl group. Examples of such a compound includetris(trimethylsilyl) borate, tris(trimethylsilyl) phosphate,tris(triethylsilyl) borate, tris(triethylsilyl) phosphate,tris(triisopropylsilyl) borate, tris(triisopropylsilyl) phosphate,tris(dimethylethylsilyl) borate, tris(dimethylethylsilyl) phosphate,tris(butyldimethylsilyl) borate, and tris(butyldimethylsilyl) phosphate.Tris(trimethylsilyl) borate is preferably used. (Time-of-flight typesecondary ion mass-analysis method)

The measuring method using the time-of-flight type mass-analysis methodwill be explained. The condition of the measurement is as follows: typeof primary ions: Bi₃ ⁺⁺, primary ion energy: 25 kV, pulse width: 4.7 ns,bunching: performed, neutralization of charge: non-neutralized, anddegree of vacuum in measurement: 4×10⁻⁷ Pa. When only the activematerial is measured, the active material is included in a resin, whichis subjected to an argon ion milling device to carry out sectionalprocessing. The processed sample subjected to the measurement. In thecase of measuring the electrode, the electrode may be used as thesample. In the case of measuring the electrode, the battery isdismounted to take the electrode out of the battery, and the electrodeis measured while preventing the electrode from being exposed, to theutmost.

(Specific Surface Area)

The specific surface area of the titanium oxide compound having aTiO₂(B) structure is preferably 5 m²/g to 100 m²/g. When the specificsurface area is 5 m²/g or more, lithium ion-absorbing/releasing sitescan be amply secured, enabling the production of a higher capacitybattery. When the specific surface area is 100 m²/g or less, coulombefficiency in charge-discharge can be improved.

(Powder X-Ray Diffraction)

Whether the titanium oxide compound has a TiO₂(B) structure or not canbe identified by powder X-ray diffraction using Cu—Kα as the lightsource.

The powder X-ray diffraction measurement may be carried out in thefollowing manner. First, an object sample is ground until the averageparticle diameter reaches about 5 μm. The average particle diameter canbe found by the laser diffraction method. The ground sample is filled ina holder part which is formed on a glass sample plate and has a depth of0.2 mm. At this time, much care is necessary to fill the holder partfully with the sample. Also, special care should be taken to avoidcracking and formation of voids caused by insufficient filling of thesample. Then, a separate glass plate is used to smooth the surface ofthe sample by sufficiently pressing the separate glass plate against thesample. Much care is taken to prevent the occurrence of cracks and voidsby a lack of the sample to be filled, thereby preventing any rise anddent from the basic plane of the glass holder. Then, the glass platefilled with the sample is mounted on the powder X-ray diffractometer toobtain a diffraction pattern by using Cu—Kα rays. Because TiO₂(B)generally has low crystallinity, some samples have weak peak intensitiesof X-ray diffraction diagram in the powder X-ray measurement so that anyone of the peak intensities is measured with difficulty. However, it isonly required to observe the peak derived from monoclinic titaniumdioxide belonging to the space group C2/m.

According to the embodiment, the electrode material which can realize anonaqueous electrolyte battery limited in the increase of resistance andimproved in cycle life can be provided

Second Embodiment

Next, a method of producing the electrode material described in thefirst embodiment will be explained. The method comprises dippingparticle of a titanium oxide compound having a TiO₂(B) structure in asolution of a compound having a trialkylsilyl group represented by theabove formula (I) and separating the particle of the titanium oxidecompound to which the above compound having a trialkylsilyl group isadhere from the solution.

wherein, R¹, R² and R³, which may be the same or different, respectivelyrepresent an alkyl group having 1 to 10 carbon atoms.

To mention in more detail, particle of titanium oxide compound having aTiO₂(B) structure is dipped in a solution of a compound having atrialkylsilyl group to impregnate the titanium oxide compound particlewith the solution. At this time, the process may be carried out undervacuum to promote the penetration of the solution into pores of thetitanium oxide compound. Then, the titanium oxide compound is separatedby, for example, filtration, followed by drying. Alternately, thetitanium oxide compound may be separated by vaporizing the solution by ahot plate or the like. By this treatment, titanium oxide compoundparticle to which the compound having a trialkylsilyl group is adheredcan be obtained.

Examples of the compound having a trialkylsilyl group include thosedescribed in the first embodiment. The content of the compound having atrialkylsilyl group may be adjusted by changing the concentration of thesolution of the compound having a trialkylsilyl group and impregnationtime.

According to the embodiment, a method of producing an electrode materialwhich can realize a nonaqueous electrolyte battery limited in theincrease of resistance and improved in cycle life can be provided.

Third Embodiment

According to this embodiment, there is provided a battery electrodecomprising an active material containing a titanium oxide compoundhaving a TiO₂(B) structure and a compound having a trialkylsilyl grouprepresented by the formula (I).

wherein, R¹, R² and R³, which may be the same or different, respectivelyrepresent an alkyl group having 1 to 10 carbon atoms.

Such an electrode may have, for example, a constitution in which anactive material layer comprising an active material containing atitanium oxide compound having a TiO₂(B) structure is formed on acurrent collector. The compound having a trialkylsilyl group may becomprised in the active material layer and is preferably dispersed inthe active material layer.

The initial charge and the subsequent charge-discharge are accompaniedby the decomposition of the nonaqueous electrolyte, leading to theformation of a coating film on the surface of the electrode. However,the presence of the compound having a trialkylsilyl group in theelectrode enables the formation of a coating film containing atrialkylsilyl group. The coating film containing a trialkylsilyl groupis stable, and can limit a side-reaction between the titanium oxidecompound and the nonaqueous electrolyte, which makes it possible tolimit the decomposition of the nonaqueous electrolyte. Excess formationof a coating film is thereby limited, and increase in the resistance ofthe battery can be limited, with result that the cycle life can beimproved.

In the electrode according to this embodiment, the coating film in whicha trialkylsilyl group is contained is not limited to the coating filmformed on the surface of the electrode but the coating film formed onthe surface of the active material comprised in the electrode.

The existence of the trialkylsilyl group in the electrode may bedetected by the time-of-flight type secondary ion mass-analysis method.Also, the existence of the trialkylsilyl group in the coating filmformed on the surface of the electrode can be detected by observing thesurface of the electrode after charge-discharge according to thetime-of-flight type secondary ion mass-analysis method.

In the electrode according the embodiment, the peak intensity ratio(I₁/I₂) is preferably 1 or more when the surface of the electrode ismeasured by the time-of-flight type secondary mass-analysis method.Here, I₁ is the peak intensity of the peak derived from Si(CH₃)₃ ⁺, I₂is the peak intensity of the peak derived from Ti⁺. The existence of thetrialkylsilyl group in the electrode increases the ratio of peakintensities to 1 or more. The existence of the trialkylsilyl group inthe coating film formed after charge-discharge also increases the ratioof peak intensities to 1 or more. The electrode having a peak intensityratio (I₁/I₂) of 1 or more is more limited in the increase of theresistance of the battery and is more improved in cycle life. In thiscase, the peak intensity ratio (I₁/I₂) is preferably 10 or less becausean excessive existence of the trialkylsilyl group is a cause of increasein resistance.

Examples of the compound having a trialkylsilyl group include thosedescribed in the first embodiment.

The electrode in this embodiment may be used either as the positiveelectrode or as the negative electrode. Preferably it is used as thenegative electrode.

Also, the electrode material described in the first embodiment may beused as the active material comprised in the electrode.

According to the above embodiment, an electrode which can realize anonaqueous electrolyte battery limited in the increase of resistance andimproved in cycle life can be provided.

Fourth Embodiment

Next, a method of producing the electrode described in the thirdembodiment will be explained. This method comprises preparing a slurryby dispersing an active material comprising a titanium oxide compoundhaving a TiO₂(B) structure and a compound having a trialkylsilyl grouprepresented by the formula (I) in a solvent.

wherein, R¹, R² and R³, which may be the same or different, respectivelyrepresent an alkyl group having 1 to 10 carbon atoms.

Then, the obtained slurry is applied to one or both surfaces of acurrent collector, dried and pressed, thereby enabling the production ofan electrode. When the slurry is prepared, other components such as aconductive agent and a binder may be added to the solution.

For example, N-methylpyrrolidone (NMP) or water may be used as thesolvent.

As the compound having a trialkylsilyl group, those described in thefirst embodiment may be used.

For example, an aluminum foil or an aluminum alloy foil may be used asthe current collector, though the material of the current collector isnot limited to these foils.

The method of producing an electrode in this embodiment may be used toany of the positive electrode and negative electrode. It is preferablyused to produce the negative electrode. The solvent, compound having atrialkylsilyl group, current collector, conductive agent, and binder arerespectively selected from the candidate which suitable to the positiveor negative electrodes.

In an example of the production of the negative electrode, for example,a powder of a titanium oxide compound having a TiO₂(B) structure,tris(trimethylsilyl) phosphate solution, acetylene black, andpolyvinylidene fluoride are added to and mixed with a NMP solvent toproduce a slurry. This slurry is applied to the current collector anddried to remove NMP, thereby enabling the production of a negativeelectrode.

According to the embodiment, a method of producing an electrode whichcan attain a nonaqueous electrolyte battery which is limited in theincrease of resistance and improved in cycle life can be provided.

Fifth Embodiment

In the fifth embodiment, a nonaqueous electrolyte battery comprising anegative electrode, a positive electrode, a nonaqueous electrolyte, aseparator and a container is provided. In this embodiment, the negativeelectrode comprises a titanium oxide compound having a TiO₂(B)structure. Also, the nonaqueous electrolyte comprises a compound havinga trialkylsilyl group represented by the formula (I).

wherein, R¹, R² and R³, which may be the same or different, respectivelyrepresent an alkyl group having 1 to 10 carbon atoms.

Hereinafter, the negative electrode, positive electrode, nonaqueouselectrolyte, separator, and container will be explained in detail.

1) Negative Electrode

The negative electrode comprises a current collector, a negativeelectrode layer (namely, a negative electrode active material-containinglayer). The negative electrode layer is formed on one or both surfacesof the current collector and comprises an active material, a conductiveagent, and a binder.

As the active material, a titanium oxide compound having a TiO₂(B)structure is used. The titanium oxide compound having a TiO₂(B)structure may be used alone, or it may be used in combination with othertitanium-containing oxides such as lithium titanate having a spinelstructure and titanium oxide having an anatase structure.

The conductive agent serves to improve current-collecting performanceand to restrain the contact resistance between the active material andthe current collector. Examples of the conductive agent includeacetylene black, carbon black, graphite, carbon nano-fiber and carbonnanotube.

The binder serves to bind the active material, conductive agent andcurrent collector with each other. Examples of the binder include apolytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF),fluoro-rubber, and styrene-butadiene rubber.

The active material, conductive agent, and binder in the negativeelectrode layer are preferably blended in ratios of 70% to 96% by mass,2% to 28% by mass and 2% to 28% by mass respectively. When the amount ofthe conductive agent is 2% by mass or more, the current collectingperformance of the negative electrode layer can be improved. Also, whenthe amount of the binder is 2% by mass or more, the binding strengthbetween the negative electrode layer and the current collector can beimproved. On the other hand, the amounts of the conductive agent andbinder are respectively preferably 28% by mass or less with the view ofdeveloping a high-capacity battery.

The current collector is preferably electrochemically stable in apotential range higher than 1.0 V and is preferably an aluminum foil oran aluminum alloy foil containing at least one element selected from Mg,Ti, Zn, Mn, Fe, Cu, and Si.

The negative electrode can be manufactured by suspending, for example,the active material, conductive agent and binder in a proper solvent toprepare slurry, by applying this slurry to the surface of the currentcollector and by drying the slurry, followed by pressing. The negativeelectrode may also be manufactured by forming a pellet essentiallyconsisting of the active material, conductive agent and binder toproduce a negative electrode layer-forming material, which is thenformed on the current collector.

2) Positive Electrode

The positive electrode comprises a current collector and a positiveelectrode layer (namely, positive electrode active material-containinglayer). The positive electrode layer is formed on one or both surfacesof the current collector and contains an active material, a conductiveagent and a binder.

As the active material, for example, oxides or sulfides may be used.

Examples of the oxides include oxides having a layer structure, such as,lithium-cobalt complex oxide (for example, Li_(x)CoO₂),lithium-nickel-cobalt complex oxide (for example,Li_(x)Ni_(1-y)Co_(y)O₂), lithium-nickel-manganese complex oxide (forexample, Li_(x)Ni_(1-y)Mn_(y)O₂), and lithium-nickel-cobalt-manganesecomplex oxide (for example, Li_(x)Ni_(1-y-z)Co_(y)Mn_(z)O₂); oxideshaving a spinel structure, such as, lithium-manganese complex oxide (forexample, Li_(x)Mn₂O₄), and lithium-manganese-nickel complex oxide(Li_(x)Mn_(2-y)Ni_(y)O₄); and compounds having an olivine structure,such as, lithium iron phosphate (for example, Li_(x)FePO₄), lithium ironphosphate manganese complex oxide (for example, Li_(x)Fe_(1-y)Mn_(y)PO₄)and lithium cobalt phosphate (for example, Li_(x)CoPO₄). Here, x and ypreferably satisfy the following equations: 0<x≦1, 0≦y≦1 and 0≦z≦1. Asthe active material, these compounds may be used alone or incombinations of two or more.

The specific surface area of the positive electrode active material ispreferably 0.1 m²/g to 10 m²/g. When the specific surface area is 0.1m²/g or more, the lithium ion-absorption/releasing sites can besufficiently secured. When the specific surface area is 10 m²/g or less,the positive electrode active material is easily handled in industrialproduction processes, ensuring good charge-discharge cycle performance.

The conductive agent improves the current collecting ability of theactive material and reduces the contact resistance between the activematerial and the current collector. Examples of the conductive agentinclude carbonaceous materials such as acetylene black, carbon black,graphite, carbon nano-fiber and carbon nanotube.

The binder serves to bind the active material, conductive agent andcurrent collector with each other. Examples of the binder include apolytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF) andfluoro-rubber.

The active material, conductive agent and binder in the positiveelectrode layer are preferably formulated in ratios of 80% to 95% bymass, 3% to 18% by mass and 2% to 17% by mass respectively. Theconductive agent can produce the aforementioned effect by blending it inan amount of 3% by mass or more. The decomposition of the nonaqueouselectrolyte on the surface of the conductive agent can be reduced byblending it in an amount of 18% by mass or less when the nonaqueouselectrolyte is stored at high temperatures. When the amount of thebinder is designed to be 2% by mass or more, sufficient strength of thepositive electrode can be obtained. When the amount of the binder is 17%by mass or less, the amount of the binder to be formulated as aninsulating material in the positive electrode can be reduced, making itpossible to reduce internal resistance.

The current collector is preferably made of an aluminum foil or aluminumalloy foil containing at least one element selected from Mg, Ti, Zn, Mn,Fe, Cu and Si.

The positive electrode can be manufactured by suspending, for example,the active material, conductive agent and binder in a proper solvent toprepare slurry, by applying this slurry to the surface of the positiveelectrode current collector and by drying the slurry, followed bypressing. The positive electrode may also be manufactured by forming apellet essentially consisting of the active material, conductive agentand binder to produce a positive electrode layer, which is formed on thecurrent collector.

3) Nonaqueous Electrolyte

For example, a liquid nonaqueous electrolyte prepared by dissolving anelectrolyte in an organic solvent or a gel-like nonaqueous electrolyteprepared by making a complex of a liquid nonaqueous electrolyte and apolymer material may be used as the nonaqueous electrolyte. In any case,the nonaqueous electrolyte comprises the compound having a trialkylsilylgroup represented by the formula (I).

Examples of the electrolyte contained in the liquid nonaqueouselectrolyte include lithium salts such as lithium perchlorate (LiClO₄),lithium hexafluorophosphate (LiPF₆), lithium tetrafluoroborate (LiBF₄),hexafluoro arsenic lithium (LiAsF₆), lithium trifluoromethasulfonate(LiCF₃SO₃), and bistrifluoromethylsulfonylimide lithium [LiN(CF₃SO₂)₂],or mixtures of these compounds. The electrolyte is preferably one whichis scarcely oxidized at a high potential and LiPF₆ is most preferable.The electrolyte is preferably dissolved in an organic solvent in aconcentration of 0.5 M to 2.5 M.

Examples of the organic solvent include cyclic carbonates such aspropylene carbonate (PC), ethylene carbonate (EC) and vinylenecarbonate; chain carbonates such as diethyl carbonate (DEC), dimethylcarbonate (DMC) and methylethyl carbonate (MEC); cyclic ethers such astetrahydrofuran (THF), 2-methyltetrahydrofuran (2MeTHF) and dioxolan(DOX); chain ethers such as dimethoxyethane (DME) and diethoxyethane(DEE); γ-butyrolactone (GBL), acetonitrile (AN) and sulfolan (SL). Theseorganic solvents may be used either alone or in combinations of two ormore.

Examples of the polymer material include a polyvinylidene fluoride(PVdF), polyacrylonitrile (PAN) and polyethylene oxide (PEO).

Examples of the compound having a trialkylsilyl group represented by theformula (I) and being comprised in the nonaqueous electrolyte includethose described in the first embodiment.

The content of the compound having a trialkylsilyl group in thenonaqueous electrolyte is preferably 0.02% to 3% by mass based on thetotal mass of the nonaqueous electrolyte. When the content is 0.02% bymass or more, the coating film containing a trialkylsilyl group isformed on the electrode, thereby obtaining a side-reaction limitingeffect. When the content is 3% by mass or less, excess formation of acoating film is prevented. The content is more preferably 0.5% to 2% bymass.

Examples of the nonaqueous electrolyte include a solution prepared bydissolving 1 M of LiPF₆ and 1% by mass of tris(trimethylsilyl) phosphatein a mixed solvent comprising ethylene carbonate and diethyl carbonatein a ratio by volume of 1:2.

4) Separator

The separator may be formed of, porous films containing, for example, apolyethylene, polypropylene, cellulose or polyvinylidene fluoride (PVdF)or nonwoven fabrics made of synthetic resins. Porous films formed of apolyethylene or polypropylene is preferably used. These porous films canmelt at a fixed temperature to cut off current and can therefore improvesafety.

5) Container

A container made of a laminate film 0.2 mm or less in thickness or ametal container 0.5 mm or less in thickness may be used as thecontainer. The metal container preferably has a thickness of 0.2 mm orless.

Examples of the shape of the container include a flat type (thin type),angular type, cylinder type, coin type and button type. As thecontainer, an appropriate one may be selected corresponding to thedimension of the battery. Containers for miniature batteries to bemounted in, for example, mobile electronic devices or package materialsfor large batteries to be mounted on two-wheel or four-wheel vehiclesare used.

As the laminate film, a multilayer film prepared by interposing a metallayer between resin layers may be used. The metal layer is preferablyformed of an aluminum foil or aluminum alloy foil to reduce the weightof the battery. For example, polymer materials such as a polypropylene(PP), polyethylene (PE), nylon and polyethylene terephthalate (PET) maybe used for the resin layer. The laminate film can be molded into adesired shape by sealing through thermal fusion.

The metal container is made of aluminum, an aluminum alloy or the like.The aluminum alloy is preferably an alloy containing one or moreelements selected from magnesium, zinc, and silicon. When the alloycontains transition metals such as iron, copper, nickel and chromium,the amount of the transition metals is preferably 100 ppm by mass orless.

The nonaqueous battery mentioned above comprises compound having atrialkylsilyl group, consequently, a coating film containing atrialkylsilyl group is formed on the surface of the electrode and activematerial by initial charge and subsequent charge-discharge. As mentionedabove, the coating film containing a trialkylsilyl group is stable andlimits a side-reaction between the titanium oxide compound and thenonaqueous electrolyte, which makes it possible to limit thedecomposition of the nonaqueous electrolyte. Excess formation of acoating film is thereby limited, so that an increase in the resistanceof the battery can be limited, with the result that the cycle life canbe improved.

The existence of the trialkylsilyl group on the coating film formedafter charge-discharge can be detected by the time-of-flight typesecondary ion mass-analysis method. The measurement may be carried outin any one of the coating film formed on the surface of the electrodeand the coating film formed on the surface of the active material. It isconvenient to measure the surface of the electrode. Hereafter the caseof measuring the surface of the electrode will be explained, but thecoating film formed on the surface of the active material can bemeasured in the same manner.

In the nonaqueous electrolyte battery according to the embodiment, thepeak intensity ratio (I₁/I₂) is preferably 1 or more when the surface ofthe electrode material is measured by the time-of-flight type secondarymass-analysis method.

Here, I₁ is the peak intensity of the peak derived from Si(CH₃)₃ ⁺, andI₂ is the peak intensity of the peak derived from Ti⁺. The existence ofthe trialkylsilyl group in the coating film formed on the electrodeincreases the ratio of peak intensities to 1 or more. The nonaqueouselectrolyte battery having a peak intensity ratio (I₁/I₂) of 1 or moreis more limited in the increase of the resistance of the battery and ismore improved in cycle life. The peak intensity ratio (I₁/I₂) ispreferably 10 or less because an excessive existence of thetrialkylsilyl group is a cause of increase in resistance.

Next, as an example of the nonaqueous electrolyte battery according tothe embodiment, a thin-type nonaqueous electrolyte battery provided witha container made of a laminate film will be explained. FIG. 1 is atypical sectional view of the thin-type nonaqueous electrolyte batteryand FIG. 2 is an enlarged sectional view of the part A shown in FIG. 1.In this case, each of these drawings is a typical view for explanationsand promotion of the understanding of the embodiment. Though there areparts different from an actual battery in shape, dimension and ratio,these structural designs may be properly changed taking the followingexplanations and known technologies into consideration.

A flat type coil electrode group 1 is accommodated in a baggy container2 made of a laminate film obtained by interposing an aluminum foilbetween two resin layers. The coil electrode groups 1 having a flat formare formed by spirally coiling a laminate obtained by laminating anegative electrode 3, a separator 4, a positive electrode 5 and aseparator 4 in this order from the outside and by press-molding thecoiled laminate. The outermost negative electrode 3 has a structure inwhich as shown in FIG. 2, a negative electrode layer 3 b is formed onone of the inside surfaces of a negative electrode current collector 3a. Other negative electrodes 3 each have a structure in which a negativeelectrode layer 3 b is formed on each surface of the negative electrodecurrent collector 3 a. An active material comprised in the negativeelectrode layer 3 b, comprises a titanium oxide compound having aTiO₂(B) structure. In the positive electrode 5, a positive electrodelayer 5 b is formed on each surface of a positive electrode currentcollector 5 a.

In the vicinity of the outer peripheral end of the coil electrode group1, a negative electrode terminal 6 is connected to the negativeelectrode current collector 3 a of the outermost negative electrode 3and a positive electrode terminal 7 is connected to the positiveelectrode current collector 5 a of the inside positive electrode 5.These negative electrode terminal 6 and positive electrode terminal 7are externally extended from an opening part of the baggy container 2. Aliquid nonaqueous electrolyte is, for example, injected from the openingpart of the baggy package material 2. The opening part of the baggypackage material 2 is closed by heat sealing with the negative electrodeterminal 6 and positive electrode terminal 7 extended out of the openingpart to thereby seal the coil electrode group 1 and liquid nonaqueouselectrolyte.

The negative electrode terminal 6 is made of, for example, a materialhaving electric stability and conductivity in a potential range from 1 Vto 3 V with respect to a lithium ion metal. Examples of the material forthe negative electrode terminal include aluminum and aluminum alloyscontaining one or more elements selected from Mg, Ti, Zn, Mn, Fe, Cu andSi. The negative electrode terminal 6 is preferably made of the samematerial as the negative electrode current collector 3 a to reduce thecontact resistance with the negative electrode current collector 3 a.

The positive electrode terminal 7 is made of, for example, a materialhaving electric stability and conductivity in a potential range from 3 Vto 4.5 V with respect to a lithium ion metal. Examples of the materialfor the positive electrode terminal include aluminum and aluminum alloyscontaining one or more elements selected from Mg, Ti, Zn, Mn, Fe, Cu andSi. The positive electrode terminal 7 is preferably made of the samematerial as the positive electrode current collector 5 a to reduce thecontact resistance with the positive electrode current collector 5 a.

According to the embodiment, a nonaqueous electrolyte battery can beprovided which is limited in the increase of resistance and improved incycle life.

In the nonaqueous electrolyte battery according to this embodiment, theelectrode material described in the first embodiment may be used as thenegative electrode active material. Alternatively, the nonaqueouselectrolyte battery may use the electrode described in the thirdembodiment as the negative electrode. In such a nonaqueous electrolytebattery, the nonaqueous electrolyte may not include the compound havinga trialkylsilyl group represented by the formula (I) though it maycontain the compound. In any of these constitutions, an increase inresistance is limited, so that the nonaqueous electrolyte battery can beimproved in cycle life.

Sixth Embodiment

Next, a battery pack according to a six embodiment will be explainedwith reference to the drawings. The battery pack comprises one or two ormore of the above nonaqueous electrolyte batteries (unit cells)according to the above third embodiment. When the battery pack includestwo or more unit cells, these unit cells are disposed in such a mannerthat they are electrically connected in series or in parallel.

FIGS. 3 and 4 show an example of a battery pack comprising two or moreflat-type unit cells. FIG. 3 is an exploded perspective view of thebattery pack. FIG. 4 is a block diagram showing an electric circuit ofthe battery pack shown in FIG. 3.

A plurality of unit cells 8 are laminated such that the externallyextended negative electrode terminals 6 and positive electrode terminals7 are arranged in the same direction and fastened with an adhesive tape9 to thereby configure a battery module 10. These unit cells 8 areelectrically connected in series as shown in FIG. 4.

A printed wiring board 11 is disposed opposite to the side surface ofthe unit cell 8 from which the negative electrode terminal 6 andpositive electrode terminal 7 are extended. As shown in FIG. 4, athermistor 12, a protective circuit 13 and an energizing terminal 14connected to external devices are mounted on the printed wiring board11. An insulating plate (not shown) is attached to the surface of theprinted wiring board 11 facing the battery module 10 to avoidunnecessary electrical connection with the wiring of the battery module10.

A positive electrode side lead 15 is connected with the positiveelectrode terminal 7 positioned on the lowermost layer of the batterymodule 10 with its tip being inserted into a positive electrode sideconnector 16 of the printed wiring board 11 for electrical connection. Anegative electrode side lead 17 is connected with the negative electrodeterminal 6 positioned on the uppermost layer of the battery module 10with its tip being inserted into a negative electrode side connector 18of the printed wiring board 11 for electrical connection. Theseconnectors 16 and 18 are connected to a protective circuit 13 throughwirings 19 and 20 formed on the printed wiring board 11.

The thermistor 12 is used to detect the temperature of the unit cell 8and the detected signals are transmitted to the protective circuit 13.The protective circuit 13 can shut off a plus side wiring 21 a and minusside wiring 21 b between the protective circuit 13 and the energizingterminal 14 connected to external devices in a predetermined condition.The predetermined condition means, for example, the case where thetemperature detected by the thermistor 12 is a predetermined one orhigher. Also, the predetermined condition means, for example, the caseof detecting overcharge, overdischarge and over-current of the unit cell8. The detections of this overcharge and the like are made forindividual unit cells 8 or whole unit cells 8. When individual unitcells 8 are detected, either the voltage of the battery may be detectedor the potential of the positive electrode or negative electrode may bedetected. In the latter case, a lithium electrode used as a referenceelectrode is inserted between individual unit cells 8. In the case ofFIGS. 3 and 4, a wiring 25 for detecting voltage is connected to eachunit cell 8 and the detected signals are transmitted to the protectivecircuit 13 through these wirings 25.

A protective sheet 22 made of a rubber or resin is disposed on each ofthe three side surfaces of the battery module 10 excluding the sidesurface from which the positive electrode terminal 7 and negativeelectrode terminal 6 are projected.

The battery module 10 is accommodated in a receiving container 23together with each protective sheet 22 and printed wiring board 11.Specifically, the protective sheet 22 is disposed on each inside surfacein the direction of the long side and on one of the inside surfaces inthe direction of the short side of the receiving container 23, and theprinted wiring board 11 is disposed on the other inside surface in thedirection of the short side. The battery module 10 is positioned in aspace enclosed by the protective sheet 22 and the printed wiring board11. A lid 24 is attached to the upper surface of the receiving container23.

Here, a thermally contracting tape may be used in place of the adhesivetape 9 to secure the battery module 10. In this case, after theprotective sheet is disposed on both sides of the battery module and thethermally contracting tapes are wound around the battery module; thethermally contracting tape is contracted by heating to fasten thebattery module.

The structure in which the unit cells 8 are connected in series is shownin FIGS. 3 and 4. However, with regard to these unit cells 8, eitherseries or series-parallel cell connections may be used to increase thecapacity of the battery. The assembled battery packs may be furtherconnected in series or parallel.

Also, the structure of the battery pack is appropriately changedaccording to its use. The battery pack is preferably used inapplications exhibiting excellent cycle performance when a large currentis extracted. Specific examples of these applications include powersources for digital cameras, and power sources mounted on vehicles suchas two- to four-wheel vehicles hybrid electric cars, two- to four-wheelelectric cars and assist bicycles. The battery pack is preferably usedfor power sources mounted on vehicles.

According to the embodiment, a nonaqueous electrolyte battery having anexcellent cycle life is provided and therefore a battery pack improvedin cycle life can be provided.

EXAMPLES Example 1 Production of a Negative Electrode

Particle of a titanium oxide compound (TiO₂) having a TiO₂(B) structurewas dipped in a tris(trimethylsilyl) phosphate solution. The solutionwas filtrated to separate the particle. Then the particle was dried. Anelectrode material was thus obtained. The specific surface area of theparticle was 13.6 m²/g.

This electrode material, acetylene black and PVdF were dissolved in NMPin a ratio by weight of 100:10:10 to prepare a negative electrodeslurry. This negative electrode slurry was applied to the surface of analuminum foil and dried to obtain a negative electrode.

<Production of an Evaluation Cell>

The negative electrode produced above, a lithium metal as the counterelectrode and a glass filter as a separator were used to produce a cell.The negative electrode and counter electrode were put in a three-poletype glass cell under the argon atmosphere in such a manner that theboth faced each other with a separator being interposed therebetween anda reference electrode made of a lithium metal was inserted in such amanner as to be in contact with neither the negative electrode nor thecounter electrode.

Each of the negative electrode, counter electrode and referenceelectrode was connected with the terminal of the glass cell and anonaqueous electrolyte was poured into the glass cell. The glass cellwas sealed in the condition that the separator and electrode weresufficiently impregnated with the nonaqueous electrolyte.

In the Example 1, as the nonaqueous electrolyte, a solution was usedwhich was prepared by dissolving 1.0 mol/L of LiPF₆ as an electrolyte ina mixed solvent obtained by mixing ethylene carbonate (EC) with diethylcarbonate (DEC) in a ratio by volume of 1:2.

Example 2 Production of a Negative Electrode

An evaluation cell was produced in the same manner as in Example 1except that a titanium oxide compound having a TiO₂(B) structure,acetylene black, PVdF and tris(trimethylsilyl) phosphate were dissolvedin NMP in a ratio by weight of 100:10:10:2 to prepare a negativeelectrode slurry.

Example 3

A titanium oxide compound having a TiO₂(B) structure, acetylene blackand PVdF were dissolved in NMP to prepare a negative electrode slurry.This negative electrode slurry was applied to the surface of an aluminumfoil and dried to obtain a negative electrode.

A mixed solvent was prepared by mixing ethylene carbonate (EC) withdiethyl carbonate (DEC) in a ratio by volume of 1:2. A nonaqueouselectrolyte was prepared by dissolving LiPF₆ in a concentration of 1.0mol/L and tris(trimethylsilyl) phosphate in a concentration of 2% bymass in the mixed solvent.

An evaluation cell was produced in the same manner as in Example 1 byusing the negative electrode and nonaqueous electrolyte prepared above.

Example 4

An evaluation cell was produced in the same manner as in Example 3except that tris(trimethylsilyl) borate was used in place oftris(trimethylsilyl) phosphate to prepare a nonaqueous electrolyte.

Example 5

An evaluation cell was produced in the same manner as in Example 3except that tris(triethylsilyl) phosphate was used in place oftris(trimethylsilyl) phosphate to prepare a nonaqueous electrolyte.

Example 6

An evaluation cell was produced in the same manner as in Example 3except that tris(triethylsilyl) borate was used in place oftris(trimethylsilyl) phosphate to prepare a nonaqueous electrolyte.

Example 7

An evaluation cell was produced in the same manner as in Example 3except that tris(butyldimethylsilyl) borate was used in place oftris(trimethylsilyl) phosphate to prepare a nonaqueous electrolyte.

Comparative Example 1

A titanium oxide compound having a TiO₂(B) structure, acetylene blackand PVdF were dissolved in NMP to prepare a negative electrode slurry.This negative electrode slurry was applied to the surface of an aluminumfoil and dried to obtain a negative electrode.

A nonaqueous electrolyte was prepared by dissolving LiPF₆ in aconcentration by volume of 1.0 mol/L in a mixed solvent obtained bymixing ethylene carbonate (EC) with diethyl carbonate (DEC) in a ratioby volume of 1:2.

An evaluation cell was produced in the same manner as in Example 1 byusing the negative electrode and nonaqueous electrolyte prepared above.

Comparative Example 2

A mixed solvent was prepared by mixing ethylene carbonate (EC) withdiethyl carbonate (DEC) in a ratio by volume of 1:2. A nonaqueouselectrolyte was prepared by dissolving LiPF₆ in a concentration byvolume of 1.0 mol/L and further vinylene carbonate in an amount of 2% bymass in the mixed solvent.

An evaluation cell was produced in the same manner as in ComparativeExample 1 except that the nonaqueous electrolyte prepared above wasused.

Comparative Example 3

An evaluation cell was produced in the same manner as in ComparativeExample 2 except that fluoroethylene carbonate was used in place ofvinylene carbonate to prepare a nonaqueous electrolyte.

Comparative Example 4

An evaluation cell was produced in the same manner as in ComparativeExample 2 except that vinylethylene carbonate was used in place ofvinylene carbonate to prepare a nonaqueous electrolyte.

(Charge-Discharge Test)

Using each evaluation cell of Examples 1 to 7 and Comparative Examples 1to 4, a charge-discharge cycle was repeated 25 and 50 times(charge/discharge operation is performed in one cycle) to examine theresistance increase ratio. The charge-discharge was carried out in thefollowing condition; charge-discharge rate: 20 mA/g and 200 mA/g andvoltage range: 1.0 to 3.0 V.

The evaluation cell was made to discharge from fully charged state atcurrent densities of 20 mA/g and 200 mA/g to calculate d.c. resistancevalue (R₁) from the cell voltage after discharged for 10 seconds. Also,resistance values (R₂₅ and R₅₀) after 25 cycles and 50 cycles werelikewise measured to calculate the increase ratio of resistance (R₂₅/R₁and R₅₀/R₁). The results are shown in Table 1. Also, a graph showing theincrease ratio in each resistance of Examples 1 to 3 and ComparativeExamples 1 to 4 is shown in FIG. 5.

(Time-of-Flight Type Secondary Ion Mass-Analysis Method)

Each evaluation cell of Examples 1 to 7 and Comparative Examples 1 to 4after 50 charge-discharge cycles were finished was subjected totime-of-flight type secondary ion mass-analysis method. The surface ofthe active material was measured in Example 1 and the surface of thenegative electrode was measured in other Examples and ComparativeExamples. The results are shown in Table 1.

TABLE 1 I₁/I₂ R₂₅/R₁ R₅₀/R₁ Example 1 3.92 1.04 1.09 Example 2 3.30 1.061.10 Example 3 2.13 1.05 1.09 Example 4 2.21 1.03 1.11 Example 5 1.991.04 1.10 Example 6 2.02 1.06 1.12 Example 7 1.34 1.05 1.11 Comparative0.24 1.08 1.19 Example 1 Comparative 0.19 1.17 1.37 Example 2Comparative 0.32 1.28 1.63 Example 3 Comparative 0.54 1.14 1.38 Example4

The ratios I₁/I₂ of peak intensities of Examples 1 to 7 were all 1 ormore, whereas the ratios of I₁/I₂ of peak intensities of ComparativeExamples 1 to 4 are all less than 1. The fact shows that the ratio I₁/I₂of peak intensities becomes 1 or more when any one of the electrodematerial, electrode, and nonaqueous electrolyte comprises the compoundhaving a trialkylsilyl group.

Also, each of Examples 1 to 7 has a lower resistance increase ratio thaneach of Comparative Examples 1 to 4, showing that the increase inresistance after charge-discharge cycles are finished is limited whenthe compound having a trialkylsilyl group is comprised in any one of theelectrode material, electrode, and nonaqueous electrolyte.

Comparative Example 2 is a type using a nonaqueous electrolytecontaining vinylene carbonate which is reported to have the effect ofrestraining the formation of a coating film in the case of using acarbonaceous negative electrode. Because the resistance increase ratioof Comparative Example 2 is higher than that of Comparative Example 1,it is shown that the effect of vinylene carbonate, that is, the effectof limiting the formation of a coating film cannot be obtained in thebattery using a titanium oxide compound having a TiO₂(B) structure asthe active material.

Comparative Example 3 is a type containing fluoroethylene carbonate in anonaqueous electrolyte. The resistance increase ratio of ComparativeExample 3 was significantly higher than that of each of ComparativeExamples 1, 2 and 4. This suggests that an inorganic coating filmcontaining LiF and the like is excessively formed by the action offluorine contained in fluoroethylene carbonate.

Comparative Example 4 is a type containing vinylethylene carbonate in anonaqueous electrolyte. This Comparative Example 4 exhibited almost thesame resistance increase ratio as Comparative Example 2. This shows thatthe effect of restraining the formation of a coating film cannot beobtained even by vinylethylene carbonate.

Comparative Example 5

An evaluation cell was produced in the same manner as in Example 1except that lithium titanate was used in place of the titanium oxidecompound having a TiO₂(B) structure.

Comparative Example 6

An evaluation cell was produced in the same manner as in Example 2except that lithium titanate was used in place of the titanium oxidecompound having a TiO₂(B) structure.

Comparative Example 7

An evaluation cell was produced in the same manner as in Example 3except that lithium titanate was used in place of the titanium oxidecompound having a TiO₂(B) structure.

(Charge-Discharge Test)

Using each evaluation cell of Comparative Examples 5 to 7, acharge-discharge test was carried out in the same manner as above tocalculate the increase ratio of resistance. The results are shown inTable 2.

TABLE 2 R₂₅/R₁ R₅₀/R₁ Comparative 1.00 1.01 Example 5 Comparative 1.001.01 Example 6 Comparative 1.00 1.01 Example 7

Comparative examples 5 to 7 all have very low resistance increase ratioafter the predetermined cycle test.

(Time-of-Flight Type Secondary Ion Mass-Analysis Method)

Each evaluation cell of Comparative Examples 5 to 7 after 50charge-discharge cycles were finished was subjected to time-of-flighttype secondary ion mass-analysis method. In the analysis method, thesurface of the negative electrode was measured. As a result, the ratio(I₁/I₂) of peak intensities each of Comparative Examples 5 to 7 were allless than 0.1.

From Comparative Examples 5 and 6, it is considered that sincetris(trimethylsilyl) phosphate was not acted on the surface of lithiumtitanate, trialkylsilyl group was not detected.

With regard to Comparative Example 7, an excessive coating film is notformed because the surface of lithium titanate has a low solid acidity.It is also considered that since tris(trimethylsilyl) phosphate was notacted on the surface of lithium titanate, trialkylsilyl group was notdetected.

Because no trialkylsilyl group was detected in Comparative Examples 5 to7, the low resistance increase ratio of Comparative Examples 5 to 7 isnot caused by the trialkylsilyl group. It is therefore shown that theeffect of limiting the increase in resistance due to the formation of acoating film containing a trialkylsilyl group is obtained in a batteryusing a titanium oxide compound having a TiO₂(B) structure.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. An electrode material comprising: an activematerial comprising a titanium oxide compound having a monoclinictitanium dioxide crystal structure; and a compound which exists on thesurface of the active material and has a trialkylsilyl group representedby the formula (I):

wherein, R¹, R² and R³, which may be the same or different, respectivelyrepresent an alkyl group having 1 to 10 carbon atoms.
 2. The electrodematerial according to claim 1, wherein when the surface of the electrodematerial is measured by the time-of-flight type secondary ionmass-analysis method, the peak intensity ratio (I₁/I₂) satisfies theequation (II):1≦I ₁ /I ₂  (II) wherein I₁ is the peak intensity of Si(CH₃)₃ ⁺ and I₂is the peak intensity of Ti⁺.
 3. A method of producing an electrodematerial, comprising: dipping particle of titanium oxide compound havinga monoclinic titanium dioxide crystal structure in a solution of acompound with a trialkylsilyl group represented by the formula (I); andseparating the particle of the titanium oxide compound to which thecompound with a trialkylsilyl group is adhere from the solution.

wherein, R¹, R² and R³, which may be the same or different, respectivelyrepresent an alkyl group having 1 to 10 carbon atoms.
 4. The methodaccording to claim 3, wherein the compound containing a trialkylsilylgroup is at least one selected from a phosphoric acid compoundcontaining a trialkylsilyl group and a boric acid compound containing atrialkylsilyl group.
 5. A battery electrode comprising: an activematerial comprising a titanium oxide compound having a monoclinictitanium dioxide crystal structure; and a compound with a trialkylsilylgroup represented by the formula (I):

wherein, R¹, R² and R³, which may be the same or different, respectivelyrepresent an alkyl group having 1 to 10 carbon atoms.
 6. The electrodeaccording to claim 5, wherein when the surface of the electrode ismeasured by the time-of-flight type secondary ion mass-analysis method,the peak intensity ratio (I₁/I₂) satisfies the equation (II):1≦I ₁ /I ₂  (II) wherein I₁ is the peak intensity of Si(CH₃)₃ ⁺ and I₂is the peak intensity of Ti⁺.
 7. A method of producing a batteryelectrode, comprising: preparing a slurry by dispersing an activematerial comprising a titanium oxide compound having a monoclinictitanium dioxide crystal structure and a compound with a trialkylsilylgroup represented by the formula (I) in a solvent:

wherein, R¹, R² and R³, which may be the same or different, respectivelyrepresent an alkyl group having 1 to 10 carbon atoms.
 8. The methodaccording to claim 7, wherein the compound containing a trialkylsilylgroup is at least one selected from a phosphoric acid compoundcontaining a trialkylsilyl group and a boric acid compound containing atrialkylsilyl group.
 9. A nonaqueous electrolyte battery comprising: anegative electrode comprising an active material which comprises atitanium oxide compound having a monoclinic titanium dioxide crystalstructure; a positive electrode; and a nonaqueous electrolyte, whereinthe nonaqueous electrolyte comprises a compound having a trialkylsilylgroup represented by the formula (I):

wherein, R¹, R² and R³, which may be the same or different, respectivelyrepresent an alkyl group having 1 to 10 carbon atoms.
 10. The batteryaccording to claim 9, wherein when the surface of the negative electrodeis measured by the time-of-flight type secondary ion mass-analysismethod, the peak intensity ratio (I₁/I₂) satisfies the equation (II):1≦I ₁ /I ₂  (II) wherein I₁ is the peak intensity of Si(CH₃)₃ ⁺ and I₂is the peak intensity of Ti⁺.
 11. The battery according to claim 9,wherein the compound containing a trialkylsilyl group is at least oneselected from a phosphoric acid compound containing a trialkylsilylgroup and a boric acid compound containing a trialkylsilyl group.
 12. Abattery pack comprising the nonaqueous electrolyte battery according toclaim 9.